1. Field of the Invention
The present invention relates generally to improvements to antennas, and more particularly to advantageous aspects of a patch antenna with a finite ground plane.
2. Description of the Prior Art
In a microstrip patch antenna, the radiator is typically provided by a metallic patch element that has been fabricated, using microstrip techniques, onto a dielectric substrate above a ground plane. Because of their low profile, low cost, and compact size, microstrip patch antennas are suitable for various microwave antenna and antenna array applications. Microstrip patch antennas are used, for example, as the radiating elements of designs based on a microwave integrated circuit (MIC) or monolithic microwave integrated circuit (MMIC) such as those used in aircraft and satellite communications, in missile and rocket antenna systems, as well as personal communication system (PCS) wireless applications. However, one problem associated with microstrip patch antennas is that they typically have a limited beamwidth, compared with, for example, antenna designs employing a dipole element. In addition, current microstrip patch antenna designs do not provide for a compact, cost-efficient mechanism for adjusting the antenna beamwidth.
The prior art can be better understood with reference to FIG. 1, which shows a cutaway perspective view of a microstrip patch antenna 10 according to the prior art. As shown in FIG. 1, the antenna 10 comprises a square patch element 12, a ground plane 14, and a microstrip feed line 16, lying on parallel planes defined by the top and bottom surfaces of a pair of dielectric substrates 18 and 20. The patch element 12 is fabricated onto the top surface of the upper substrate 18, the ground plane 14 is fabricated between the bottom surface of the upper substrate 18 and the top surface of the lower substrate 20, and the feed line 16 is fabricated onto the bottom surface of the lower substrate 20. A fixed metal plate reflector 22 is provided at the bottom of the antenna 10 to reflect radiation towards the top of the antenna 10. Coupling between the feed line 16 and the patch element 12 is provided by a small rectangular aperture 24 in the ground plane 14 that lies across the feed line 16. Because of this coupling technique, the design shown in FIG. 1 is known as an xe2x80x9caperture-coupled patch antenna.xe2x80x9d Other designs are also used, employing different techniques to couple the feed line to the patch element.
In current aperture-coupled patch antenna designs, the ground plane 14 is significantly larger than the aperture 24 such that, from an electromagnetic perspective, the ground plane 14 functions as an infinite surface relative to the aperture 24. This helps the isolation between the feed line 16 and the patch element 12. In addition, the use of an infinite ground plane makes analysis of the antenna much easier because the equivalence theorem can be applied.
An antenna""s radiation pattern is important in antenna applications. It includes several parameters to characterize the antenna performance, including gain, 3 dB (half-power) beamwidth, side-lobe level, front-to-back (F/B) ratio, polarization, cross-polarization level, and the line. The 3 dB beamwidth parameter is the main parameter to show the coverage of radiated energy. The beamwidth of a conventional patch antenna is approximately 60xc2x0 to 70xc2x0.
Because of their high level of integration, patch antennas have been used successfully to form large arrays for highly directional applications. However, other applications require a beam width of greater than the currently available 60xc2x0 to 70xc2x0. For example, a typical three-section cellular system needs to cover a 120xc2x0 geographic area. In a time division multiple access (TDMA) system, the base station requires an antenna with a 3 dB beamwidth of 105xc2x0 to 110xc2x0, and a code division multiple access (CDMA) system requires a 3 dB beamwidth of 90xc2x0. Because of the beamwidth limitations of conventional patch elements, a dipole element is typically used instead in these applications.
In addition, it is desirable for the beamwidth of an antenna to be adjustable in certain applications. A dipole element with an angular reflector can be employed to provide beamwidth control by mechanically adjusting the reflector angle. However, this approach requires sophisticated mechanical structures which may not be cost effective, and which may also result in an undesirably large package size to accommodate these structures.
One aspect of the invention provides a microstrip patch antenna with enhanced beamwidth characteristics. In a first embodiment, the antenna comprises a patch element and a ground plane separated from the patch element by a first dielectric layer. The antenna further includes a signal feed line separated from the ground plane by a second dielectric layer, the signal feed line being shielded from the patch element by the ground plane. The signal feed line is electromagnetically coupled to the patch element through an aperture in the ground plane lying across the signal feed line, the ground plane functioning as a finite surface relative to the aperture. According to a further aspect of the invention, the beamwidth of the antenna is adjusted by adjusting the position of a reflector behind the signal feed line. Thus, the present invention provides an efficient way to achieve adjustable wide-beamwidth that may be used, for example, in wireless systems in a three-sector configuration.
Additional features and advantages of the present invention will become apparent by reference to the following detailed description and accompanying drawings.